1. Field of the Invention
This invention pertains to the field of implantable medical devices, and in particular to implantable drug dispensers.
2. State of the Prior Art
Various types of drug delivery systems are well known in the prior art. The simplest and most common of these systems employs an elevated container attached to a tube which is coupled to a needle inserted into the patient's body. In such a system the rate of flow is controlled by a valve which sets the drip rate of the drug from the container into the tube. This primary disadvantage of this system is the mobility limitation imposed on patients.
Recently, much progress has been made toward the production of completely implantable infusion systems. Most such systems employ a reservoir for containing the drug, a tube leading from the reservoir to a delivery site, and a valve controlling the rate of flow of the drug into the body. Some systems rely on simple diffusion of the drug, however, most systems employ a pumping mechanism to force the drug into the delivery site. Miniaturized roller pumps and pressurized reservoirs are two common approaches to providing such a pumping force.
Common to all of the implantable systems is the need to refill the reservoir at regular intervals. Typically this is accomplished by providing the reservoir with a puncturable septum and mounting the reservoir immediately under the skin so that it may be filled by hypodermic syringe. The necessity to regularly refill the reservoir has an inherent disadvantage in that each time the hypodermic needle pierces the skin and enters the reservoir, it carries with it minute amounts of tissue and debris which contaminate the drug supply and tend to clog the fluid passageways within the dispenser.
In order to contain a sufficient quantity of the drug to provide for release over an extended period of time, the prior art devices are generally filled with a drug in concentrated form. The concentrated form of the drug in some cases leads to crystallization of the drug and blockage of the fluid passageways within the dispenser.
Recently, research has been undertaken which is directed toward the use of body cells cultured outside the body to produce drugs such as insulin. Such research is described in the article "A Hybrid Artificial Pancreas" by Chick, et al., vol. XXI Trans. Amer. Soc. Artif. Int. Organs, 1975. This article discloses that beta cells may be cultured on semipermeable membranes of a type used for artificial kidneys. The particular membrane utilized is fabricated of XM-50 acrylic copolymer with a maximum molecular weight of 50,000 and is manufactured by Amicon Corp. of Lexington, MA. These membranes are permeable to glucose and insulin, as well as to oxygen, carbon-dioxide and water.